A lithium-ion battery having a wound electrode assembly is generally fabricated in the following order. First, the wound electrode assembly is placed in an outer casing. Next, an electrolyte solution is injected into the outer casing. Thereafter, an opening of the outer casing is closed with a sealing plate.
In order for the lithium-ion battery to have a high energy density, a very large volume inside the outer casing is occupied by the electrode assembly. Therefore, the electrolyte solution injected into the outer casing is not readily impregnated into the electrode assembly. That is, the step of injecting the electrolyte solution into the outer casing requires a lot of time. If the time taken by the injection step is increased, it becomes essential to augment production equipment to carry out the injection step. This results in the need of an additional space for installation of production equipment, or leads to increase in equipment cost. Furthermore, since the electrolyte solution is evaporated into the air, the loss of the electrolyte solution also becomes large as the time taken by the injection step is increased.
A method as described below has been proposed in order to reduce the time required for the injection step. Specifically, the pressure of the environment in which an electrode assembly is placed is decreased after an electrolyte solution is injected into an outer casing. Thus, the air inside the electrode assembly and the electrolyte solution are smoothly replaced with each other, and the electrolyte solution is impregnated into the electrode assembly in a relatively short time. However, this method requires equipment for vacuuming, and therefore cannot be expected to reduce the space and cost for production equipment.
In addition, the viscosity of an electrolyte solution itself is also one of the causes that hinder the impregnation of the electrolyte solution. For example, Patent Literature 1 discloses a method in which a plurality of types of electrolyte solutions having different viscosities are prepared, and the electrolyte solutions are injected into an outer casing in order of increasing viscosity. Not only does this method require a plurality of types of electrolyte solutions, this method also requires equipment for injecting the plurality of types of electrolyte solutions into the outer casing in order. Therefore, also with this method, reduction in the space and cost for production equipment can scarcely be expected. Since a plurality of types of electrolyte solutions need to be prepared, there is also a possibility that a problem arises in acquiring the materials.